drm/i915/glk: Update Port PLL enable sequence for Geminilkae

Add steps for enabling and disabling Port PLL as per bspec.

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480667037-11215-6-git-send-email-ander.conselvan.de.oliveira@intel.com